The conventional method of reversing the direction of scan for a sector scan radar has been to apply a positive voltage to produce a scan in one direction and apply a negative voltage to produce a scan in the other direction. Previous scan systems have been overly complex using large amounts of power and requiring close synchronization among the rotating elements. This prior technique of antenna scanning is known as "plugging". Plugging places high stress on the electric drive circuits and induces demagnetization of the permanent magnets found in the stator. That is, the field in the motor armature opposes the stator field which causes the magnets of the stator to demagnetize.
The loss in magnetization is accompanied by a corresponding loss in torque. Plugging, therefore requires a large-size motor in order to accomplish rapid reversal of direction.
Some prior art optical scanners and radar scanners, although employing spring mechanisms to assist the scan reversing, have not solved the aforementioned problems. For example, U.S. Pat. No. 3,734,591 describes an optical scan apparatus wherein spring energy provides a one-way mirror scan with a rapid return feature. No provision is made for eliminating inordinate surges of current occurring during scan reversal.